Apparatus and Method for Applying Traction to an Elongate Element Produced by Fusing a Preform of Glass Material and Usable in a Process for Producing an Optical Fibre

ABSTRACT

Apparatus for applying traction to an elongate cylindrical element produced by fusion of an end portion of a preform of glass material, in which a traction device is capable of being connected to a portion of the elongate cylindrical element to provide traction of the elongate cylindrical element along an axis. A device for the rotation of the elongate cylindrical element applies a twist to the elongate cylindrical element about the axis simultaneously with the traction.

The present invention relates to an apparatus and method for applyingtraction to an elongate element which is produced by fusing a preform ofglass material and is usable in a process for producing an opticalfibre.

As is known, an optical fibre is produced by a process of drawing apreform of glass material. In particular, there is a known method ofplacing the preform in a vertical position inside a furnace to cause thefusion of a lower portion of the preform. The fused material is thenstretched downwards by a traction device, thus producing a threadlikeelement which forms the optical fibre.

The patent EP 367871 in the name of Corning Glass Works describes amethod for making an optical fibre of the step index type, having asharp change in the refractive index between the core and the cladding.This method comprises an initial stage of depositing particles of glasscomprising a base glass and a refractive index-increasing dopant on asupport (“mandrel”). The mandrel is then removed and the resultingpreform (“soot preform”) is consolidated in such a way as to form a corepreform having a surface region with a low dopant content. The corepreform is stretched and the hole present in it is closed in such a wayas to form a core bait rod. The cladding glass soot is then deposited onthe core rod at a density of at least 0.5 g/cc. This is done bydirecting the flame of an auxiliary burner onto the core bar immediatelybefore the cladding soot is deposited on it. In this way, a finalpreform is produced, and this is consolidated and drawn to produce anoptical fibre.

A process of the type described above for producing a final preform iscommonly known as the OVD (outside vapour deposition) process.

Also according to the patent EP 367871, the core preform is stretched byusing a traction device comprising a pair of powered traction wheels(indicated by 52 in FIG. 3 of the patent in question) which apply adownward traction on opposite sides of the core rod.

The applicant has noted that, in the execution of this last stage, it ispossible that, as a result of an imprecise positioning and/or operationof the traction wheels, opposite sides of the core rod undergo differentstresses which cause the core rods to bend during its forming process;therefore the production of core rods which are not perfectlyrectilinear, in other words which have shape defects, is possible.

The applicant has also noted that the bending of the core rod may alsobe introduced by non-uniformities of the temperature within the verticalfurnace.

GB 1315447 relates to an apparatus adapted for drawing pipes or bars,particularly quartz ones, from blanks of large diameter, and addressesthe problem of sagging of the pipe in a heating cone of a knownapparatus, which causes a general deformation of the finished quartzpipe, as well as an inadequate fusion of the pipe. The apparatus of GB1315447 comprises a head adapted for feeding a blank to a system of gasburners, a head for drawing a finished pipe, a drive with a shaftservicing both heads and providing for rotational movement of the pipeand the blank, and two further drives servicing both heads and providingfor axial movement of the pipe.

The applicant has observed that the aforesaid shape defects may lead tothe presence of a non-negligible error of straightness of the core rodduring the final stage of chemical deposition.

The applicant has therefore noted that these shape defects may lead tothe production of a final preform (from which the optical fibre willsubsequently be drawn) which is non-uniform, in other words a preformwhose central portion (formed by the core rod) is curved and is at anon-constant radial distance from the outer surfaces of the preform; inother words, if a cross section of the preform is viewed, it may be seenthat the central circular portion corresponding to the section of thecore rod is not concentric with the circular section corresponding tothe section of the final preform. This error of concentricity ismaintained during the drawing stage, and the optical fibre which isproduced therefore has a core which is not concentric with the cladding.

The core/clad concentricity is a basic parameter of an optical fibre,since it provides a measurement of the extent to which the axis of thecore is aligned with the axis of the cladding. More particularly, theconcentricity is defined as the distance between the axes of the coreand the cladding. Typically, the concentricity must be small (forexample, less than 0.5 μm and preferably less than 0.3 μm), so that,when two end portions of two different optical fibres are joined, theattenuation of the transmitted light is low. This is because opticalfibres are typically joined by aligning the outer surfaces of theircladdings, and therefore, if the cores are not perfectly positionedalong the axes of the corresponding fibres, the joint between the twocores may be partial, producing a high-loss joint.

The present invention makes it possible to mitigate the problem,observed by the applicant in the use of traction devices or apparatus ofknown types, of the presence of shape defects of the elongate elementwhich forms the core rod.

The applicant has found that, by using an apparatus comprising atraction device for stretching downwards the elongate element whichforms the core rod, together with a rotation device for applying a twistto the elongate element about its axis during the traction, it ispossible to produce an elongate element with a high degree ofstraightness.

The rotation device and the traction device therefore form a system forthe traction and movement of the elongate element. The rotation devicemay, for example, comprise a rotatable member carrying the tractiondevice, which in turn comprises, for example, a pair of pulleys whichcan be connected by a suitable mechanism to the elongate element. Inanother embodiment, the rotation device comprises a powered rotatablebody which can be connected by means of a mandrel to the elongateelement, and is mounted on a vertically movable slide.

The device according to the present invention is therefore suitable forthe production of intrinsically straight core rods which assist in theformation of a homogeneous final preform; this final preform, after thedrawing process, is suitable for generating an optical fibre in whichthe core axis is aligned with the cladding axis.

In a first aspect, the present invention relates to a method of applyingtraction to an elongate cylindrical element produced by the fusion of apreform of glass material, the said cylindrical element being usable ina process of producing an optical fibre, comprising the stages ofproviding traction of the elongate cylindrical element along an axis ofadvance and imparting a twist to the elongate cylindrical element aboutthe said axis of advance.

Preferably, the said stages of providing traction and imparting a twistare carried cut simultaneously.

The said stage of providing traction can comprise the stages ofconnecting a body to a portion of the said elongate cylindrical elementand providing a motion of translation of the said body along the saidaxis of advance; the said stage of imparting a twist can comprise thestage of providing the rotation of the said body about the said axis ofadvance.

The said stage of imparting a twist can also comprise the stages ofconnecting an auxiliary body to a portion of the said elongatecylindrical element when the said body reaches a limit position,disconnecting the said body from the said elongate cylindrical element,and providing a motion of translation and rotation of the said auxiliarybody along and about the said axis of advance.

In a possible variant, the said stage of providing traction can comprisethe stages of connecting at least a first and a second traction pulleyto the said elongate element and causing the said first and secondtraction pulleys to rotate about respective axes; the said tractionpulleys lie in a common plane and the said stage of imparting a twistcan comprise the stage of rotating the said plane in which the pulleyslie about the said axis of advance.

In a further aspect, the present invention relates to an apparatus forapplying traction to an elongate cylindrical element produced by fusionof a preform of glass material, the said cylindrical element beingusable in a process of producing an optical fibre, the said apparatuscomprising a traction device capable of being connected to at least aportion of the said elongate cylindrical element to provide traction ofthe elongate cylindrical element along an axis of advance, andadditionally comprising a rotation device capable of applying a twist tothe elongate cylindrical element about the said axis of advancesimultaneously with the said traction.

The said traction and twisting devices can comprise at least one bodycapable of being connected to the said portion of the said elongatecylindrical element and movable both with a rectilinear motion along thesaid axis to provide the said traction and with a rotary motion aboutthe said axis to provide the said twist.

The said traction device can also comprise a slide carrying the saidbody and movable along a guide under the force of a first motor.

The said rotation device can comprise a second motor connected to thesaid body to cause the said body to rotate about the said axis ofadvance and with respect to the said slide.

The said first motor preferably has an output shaft connected to a screwand the said slide is advantageously provided with a nut engaged withthe said screw to provide the movement of the said slide along the saidguide under the force of the said first motor.

The said body is preferably integral with a pulley connected by means ofa belt to an output shaft of the said second motor.

There may also be an auxiliary traction apparatus in which an auxiliarytraction device and an auxiliary rotation device are capable ofoperating alternately with the said traction device and with the saidrotation device to provide, respectively, the said traction and the saidtwist of the said elongate cylindrical element.

The said auxiliary traction device and the said auxiliary rotationdevice preferably comprise at least one auxiliary body which is capableof being connected to the said portion of the said elongate cylindricalelement and is movable both with a linear motion along the said axis ofadvance to provide the said traction and with a rotary motion about thesaid axis of advance to provide the said twist.

In a possible variant, the said traction device can comprise at leastone pair of powered traction pulleys capable of being connected to atleast one portion of the said elongate cylindrical element and ofproviding traction of the elongate cylindrical element along the saidaxis of advance; the said rotation device being capable of providing arotary motion of the said traction pulleys about the said axis ofadvance to provide the said twist of the said elongate cylindricalelement.

The apparatus can also comprise a positioning device capable of placingthe said traction pulleys in at least two positions, including:

an activation position in which the traction pulleys are essentiallytangential to each other and tangential to the axis of advance; and

a rest position in which the said traction pulleys are spaced apart fromeach other.

The said positioning device can comprise a first oscillating bodysupporting a first traction pulley and a second oscillating bodysupporting a second traction pulley; the said first oscillating body andthe said second oscillating body being angularly movable aboutrespective axes of rotation to place the said traction pulleys in thesaid activation position arid in the said rest position.

The said positioning device can comprise a pair of rectilinear elementshaving first end portions hinged to corresponding oscillating elementsand second end portions hinged together on an axially movable rod; thesaid positioning device additionally comprising a striking wall movableunder the force of at least one actuator and capable of bearing on thesaid rod to cause an axial movement of the said rod and, consequently, arotation of the said oscillating elements.

There may also be an elastic device interacting with the said rod tokeep the said rod in a stable position as a result of which the saidtraction pulleys are located in the said activation position.

The said rotation device can comprise a rotating equipment supportingthe said pair of traction pulleys and rotationally movable under theforce of a first motor about the said axis of advance.

The said rotating equipment can carry a first ring gear engaging with afirst gear wheel which can be driven by the said first motor to causethe said rotating equipment to rotate.

The said rotating equipment can also comprise a device for transmittingthe rotary motion from a second motor to the said pair of tractionpulleys.

The rotary motion transmission device preferably comprises a second ringgear having external toothing which engages with a second gear wheelkeyed on the output shaft of the second motor; the said ring gear alsohaving internal toothing which engages with a third gear wheel fromwhich extends a main shaft which transmits the rotary motion to the saidfirst and said second traction pulley.

The said rotary motion transmission device can also comprise:

a first bevel gear located on one end of the said main shaft;

a second bevel gear which engages with the first bevel gear and isintegral, by means of a first shaft placed perpendicularly to the axisof advance, with a first toothed pulley,

a second toothed pulley connected to the first toothed pulley; the saidsecond toothed pulley being immovably fixed to a first end of a secondshaft having a second end which carries a fourth gear wheel engaged witha fifth gear wheel located on a third shaft carried by a firstoscillating element; the said third shaft carrying the said firsttraction pulley;

the said second toothed pulley transmitting its rotary motion to a thirdtoothed pulley which is connected to a fourth toothed pulley located onthe first, end of a fourth shaft; a second end of the fourth shaftcarrying a second traction pulley.

The present invention also relates to equipment for producing anelongate cylindrical element usable in a process of producing an opticalfibre, comprising:

a furnace capable of housing a preform of glass material;

a traction apparatus comprising a traction device for stretching thesaid elongate cylindrical element from the said partially fused preformand along an axis of advance; in which the said traction apparatusadditionally comprises a rotation device capable of imparting a twist tothe elongate cylindrical element about the said axis of advance,simultaneously with the said traction.

In a further aspect, the present invention relates to a method ofproducing an elongate cylindrical element of glass material usablesubsequently for the production of an optical fibre, comprising thestages of heating an end portion of a preform of glass material in sucha way as to obtain fused glass material, exerting traction on the saidglass material to form the said elongate cylindrical element and toadvance the elongate cylindrical element along a predetermined axis, andimparting to the said elongate cylindrical element a twist about thesaid axis.

Preferably, the said stages of exerting traction arid imparting a twistare carried out simultaneously.

Further details can be found in the following description, which refersto the attached figures listed below:

FIG. 1 shows, in a front view, a traction device made according to thedictates of the present invention;

FIG. 2 shows, on an enlarged scale, a detail of the device of FIG. 1;

FIG. 3 shows, in a front view, a first variant of the device of FIG. 1;

FIG. 4 shows, in a side view, a second variant of the traction device ofFIG. 1;

FIG. 5 shows, in a front view, the traction device of FIG. 4;

FIG. 6 shows, in a view from above, a portion of the traction device ofFIG. 4;

FIG. 7 shows, in a view from above, the traction device of FIG. 4; and

FIG. 8 shows, in a simplified perspective view, a transmission device ofthe traction device of FIG. 4.

An OVD process for making a preform from which an optical fibre is to bedrawn comprises, briefly, the following stages:

-   -   I. a first stage during which a plurality of chemical substances        are deposited, by a chemical deposition process using a burner,        on a rectilinear cylindrical mandrel made from ceramic material.        The substances deposited on the mandrel mainly comprise silicon        oxide (SiO₂) typically doped with other compounds, such as        germanium oxide (GeO₂) . The product of this first stage is a        cylindrical preform (core preform) of glass material which will        subsequently form the core of the optical fibre;    -   II. a second stage in which the rectilinear ceramic mandrel is        extracted from the preform, leaving a central hole in the        preform;    -   III. a third stage in which the previously made preform is        subjected to a process of drying and consolidation in a furnace        in order to eliminate the hydroxide ions (—OH) and the atoms of        water present in the preform; thus a vitrified preform is        obtained, still having a central hole;    -   IV. a fourth stage in which, after the central hole of the        preform has been closed (with a plug, for example) and a vacuum        has been created inside it, the vitrified preform (whose        diameter is typically of the order of 50-100 mm) is placed in a        vertical furnace in which the fusion of a lower end of the        preform is carried out. The fusion of the lower end causes the        walls of the hole to collapse because of the vacuum created in        the hole. The fused glass material cools, forming an elongate        cylindrical element (also called the core rod) of predetermined        diameter, which is stretched downwards by a traction device.        This elongate cylindrical element is then cooled further and cut        transversely at a number of equidistant points in such a way as        to form a plurality of elongate elements, also known by the term        “canes” (whose lengths are typically of the order of a metre);    -   V. a fifth stage in which each elongate element is subjected to        a process of chemical deposition (“overcladding”), using a        burner to deposit on the elongate element a plurality of        chemical substances which will subsequently form the “clad”, or        “cladding”, of the optical fibre. A substance typically used to        produce the core is silicon oxide (SiO₂). The product of the        fifth stage is a low-density final cylindrical preform; and    -   VI. a sixth stage in which the low-density final cylindrical        preform is dried and consolidated by the same procedures as        those specified for the third stage. Thus a vitrified final        preform is obtained; this is subsequently subjected to a drawing        process similar to the process described in the fourth stage, to        produce a threadlike element of glass material forming the        optical fibre.

In FIG. 1, the number 1 indicates, as a whole, a fraction apparatuscapable of being used in the fourth stage of the aforesaid process, inequipment for producing an elongate cylindrical element usable in aprocess of producing an optical fibre. In particular, the apparatus 1 iscapable of carrying out the stretching of a cylindrical preform 2 madefrom glass material to obtain an elongate cylindrical element 5 with alength of several metres and a predetermined diameter (typically in therange from 5 mm to 20 mm), coaxial with a vertical axis 6, forming anaxis of advance in the process. The preform 2 consists principally ofsilica and suitable dopants (selected according to the transmissioncharacteristics specified for the optical fibre) and has a diameter inthe range from approximately 30 mm to 120 mm.

The apparatus 1 comprises a supporting structure 10, a traction device 3and a rotation device 4. The apparatus cooperates with a verticalfurnace 7 (which is shown schematically and is of a known type) formingpart of the aforesaid equipment.

The furnace 7 forms a cylindrical cavity within which the preform 2 ishoused coaxially. The vertical furnace 7 is capable of heating a lowerend portion (“neckdown” ) 2 a of the preform 2, thus achieving thefusion of this lower portion 2 a. More particularly, a fused portion ofglass material flows from the lower end portion 2 a towards theapparatus 1, forming the aforesaid elongate cylindrical element 5. Thelatter comprises an upper portion 5 s which is close to the furnace 7and is still in a plastic state, and a lower portion 5 i which isconnected to the apparatus 1 and is in the glassy state.

The supporting structure 10 (made from metal, for example) is locatedunder the vertical furnace 7 and is carried by a supporting frame 11(shown schematically).

The supporting structure 10 can comprise a vertical flat rectangularwall 12 and two horizontal flat rectangular walls 14, 15 which extendperpendicularly to the wall 12 along shorter edges of the latter.

The traction device 3 comprises a vertical rectilinear guide 17 carriedby the supporting structure 10 and extending between the walls 14 and15, and a slide 19 slidable with reversible motion along the verticalguide 17. In particular, the guide 17 can comprise a pair of rectilinearcylindrical bars 21, 22 which are parallel to each other and extendbetween the walls 14, 15 perpendicularly to these walls.

The slide 19 can comprise a flat wall 24 provided with two circularthrough holes, within each of which a cylindrical tubular sleeve 25 isfitted immovably so that it is perpendicular to the wall 24 and slidablealong a corresponding bar 21, 22.

The slide 19 is movable along the guide 17 under the force of anelectric motor 27, which is, for example, fixed to the wall 15. Themotor 27 preferably has an output shaft immovably fixed to a screw 30which extends parallel to the bars 21, 22 between the walls 14 and 15.Preferably, the screw 30 is engaged with a nut 32, particularly acirculating ball nut, located in a through hole of the flat wall 24 andfixed immovably to the latter in such a way that the angular rotation ofthe screw 30 produces the linear movement of the slide 19 along thevertical guide 17.

In particular, the slide 19 is movable between an upper limit position(not shown) at which the wall 24 is close to the wall 14 and a lowerlimit position (not shown) at which the wall 24 is close to the wall 15.

The wall 24 also has a further circular hole 34 coaxial with thevertical axis 6 and capable of allowing the passage of the elongatecylindrical element 5. In particular, as shown in FIG. 2, the circularhole 34 houses a powered rotatable body 40 capable of being connected inan angularly immovable way with the elongate cylindrical element 5 forimparting a twist to the elongate cylindrical element 5 about the axis6.

The rotatable body 40 can comprise a chuck 42 of a known type (of theself-centring three-jaw type, for example) which is coaxial with theaxis 6, housed partially within the circular hole 34 and connected tothe wall 24 with the interposition of a bearing 44. A lower portion ofthe chuck 42 facing the wall 15 projects from the hole 34, while anupper portion of the chuck 42, facing the wall 14, projects from thehole 34 and is connected immovably to a pulley 46.

The rotation device 4 comprises an electric motor 48 fixed to a bracket49 fixed to the wall 24 of the slide 19. The output shaft of theelectric motor 48 carries a pulley 51 connected, by means of a toothedbelt 50, to the pulley 46 to cause the chuck 42 to rotate about thevertical axis 6.

The electric motors 48 and 27 are controlled by an electronic controlunit 55 (FIG. 1) which can receive at its input, among other elements, asignal Φ correlated with the diameter of the elongate cylindricalelement 5, for example a signal generated by an opto-electronic sensor60 placed along the cylindrical element 5 in the proximity of its upperportion 5 s. An example of a sensor for measuring the diameter of theelongate cylindrical element 5 is provided in the patent U.S. Pat. No.5,314,517.

In operation, after the furnace 7 has been turned on, the slide 19 iscarried towards the upper limit position, the chuck 42 is opened and thesolidified lower portion 5 i is made to enter it. The chuck 42 is thenclosed on the portion 5 i, establishing an angularly immovableconnection between the chuck 42 and the elongate cylindrical element 5.The motor 27 is then turned on, causing the slide 19 to move towards thelower limit position, and then causing the traction (and consequentelongation, or “stretching”) of the elongate cylindrical element 5downwards at constant velocity.

The traction velocity is preferably in the range from 5 to 70 cm/min,more preferably from 15 to 35 cm/min. This range of traction velocitiesis selected according to the diameter of the elongate cylindricalelement 5 which is to be obtained.

Simultaneously with the turning on of the motor 27, the motor 48 isturned on and causes the chuck 42 to rotate in such a way that theelongate cylindrical element 5 undergoes a twist about the axis 6 duringits rectilinear linear motion along the axis 6.

The velocity of rotation of the chuck 42 is preferably such as to causea twist of the elongate cylindrical element 5 in the range from 20 to100 revolutions per metre, or more preferably from 20 to 60 revolutionsper metre.

The twist of the elongate cylindrical element 5 is transferred to thefused lower end (“neckdown”) 2 a of the preform 2.

In other words, the apparatus 1 applies a torque to the elongatecylindrical element 5 in such a way that the latter undergoes apredetermined twist about the axis 6 during its formation.

The applicant has found that, owing to the simultaneous motions oflinear movement along the axis 6 and twisting about the axis 6 appliedto the elongate cylindrical element 5, the elongate cylindrical element5 is intrinsically straighter than elongate cylindrical elementsproduced by traction apparatus of known types.

In particular, the elongate cylindrical element 5 made according to theapparatus of the present invention shows a maximum offset which is inall cases less than the offset shown by an elongate cylindrical elementformed by a traction apparatus of a known type.

For example, the applicant has observed that an elongate cylindricalelement with a length of approximately 100 cm and a diameter ofapproximately 10 mm, produced by a traction apparatus of the doublepulley type, typically shows an offset in the range from 0.2 mm to 1 mm,while an elongate cylindrical element of equal length and diameterproduced by means of the apparatus 1 typically shows an offset in therange from 0.05 mm to 0.2 mm.

The elongate cylindrical element 5 is then cut transversely at a numberof points to form a plurality of elongate cylindrical elements known as“canes” (a cane typically has a length of the order of one metre). Fromfour to eight canes are typically obtained from a preform of theconventional type.

Each cane is then subjected to an overcladdinq stage of an OVD process,by means of which the cane is covered with a layer of glassy substances,particularly silicon oxide, SiO₂. In this way a final preform isobtained, from which an optical fibre (not illustrated) is produced bydrawing, the core of this fibre originating from the elongatecylindrical element 5, while its cladding originates from the layerdeposited during the aforesaid overcladding stage.

The applicant has also observed a considerable improvement in theconcentricity of optical fibres made from canes obtained by means of atraction device according to the present invention. In particular,optical fibres having values of concentricity below 0.5 μm wereobtained.

If the length of the elongate cylindrical element 5 is essentially equalto the length of the guide 17, the apparatus 1 can carry out a singlestage of stretching the cylindrical element 5, in which the slide 19moves downwards from the upper limit position to the lower limitposition. Alternatively, If the length of the elongate cylindricalelement 5 is greater than the length of the guide 17, the apparatus 1can carry out a plurality of successive stretching stages, at the end ofeach of which the slide 19 is repositioned from the lower limit positionto the upper limit position. In practice, after the slide 19 has reachedthe lower limit position, the chuck 42 is opened, the slide 19 is movedupwards to the upper limit position, and the chuck 42 is again connectedto the elongate cylindrical element 5.

To ensure the continuity of stretching of the elongate cylindricalelement 5 during the repositioning of the slide 19, and particularly inorder to prevent a stop of the elongate cylindrical element 5 along theaxis 6 from causing an accumulation of fused material below the neckdown(and therefore a discontinuity in the diameter of the elongatecylindrical element 5), it is possible to provide (FIG. 3) an auxiliarytraction device 1 a located under the apparatus 1 and having a structureand operation completely identical (corresponding parts being indicatedby the same numbers plus the subscript “a”) to those of the apparatus 1.The apparatus 1 and 1 a are both aligned with the axis 6.

In particular, the apparatus 1 a comprises a traction device 3 a and arotation device 4 a similar to those of the apparatus 1. Accordingly,the traction device 1 a comprises a slide 19 a which is initiallylocated at the upper limit position and is provided with a chuck 42 awhich can be connected to the elongate element 5. When the slide 19 hasreached the lower limit position and can no longer carry out thetraction of the elongate cylindrical element 5, the chuck 42 a isconnected to the cylindrical element 5, the chuck 42 is opened and theslide 19 a is moved towards its lower limit position to carry out thetraction of the element 5 along the axis 6. In the same way as theapparatus 1, the apparatus 1 a produces a twist of the element 5simultaneously with its traction.

When the slide 19 a reaches the lower limit position, the operations oftraction of the element 5 are again carried out by the slide 19 which islocated near its upper limit position.

In this case, the apparatus 1 and 1 a are preferably controlled by asingle electronic control unit 55 to which are sent, among otherelements, signals detected by sensors 61, 61 a which detect the lowerlimit positions of the slides 19 and 19 a respectively.

The limit position sensors 61, 61 a can be of the mechanical type (forexample, a switch which is closed or opened by the slide 19, 19 a whenit is in the limit position) or a sensor of the opto-electronic type(for example, a photocell whose signal is interrupted by the slide 19,19 a when it is in the lower limit position).

In the present case, therefore, the traction device comprises a pair ofpowered slides 19, 19 a and respective connecting bodies (chucks) 42, 42a, while the rotation device comprises the bodies 42, 42 a, therespective motors 48, 48 a and the corresponding elements fortransmitting the motion.

FIG. 4 shows a further variant 1 b of the apparatus according to theinvention, comprising a supporting structure 62, a traction device 3 band a rotation device 4 b.

The supporting structure 62 (made from metal, for example) comprises avertical flat wall 63 and a horizontal flat wall 64 extendingperpendicularly from a central portion of the vertical wall 63. Thehorizontal wall 64 is also supported by a pair of brackets 65, which aretriangular in this particular case, fixed to the vertical wall 63.

The horizontal wall 64 has a circular central aperture 67 coaxial withan axis 6 a (axis of rotation). The rotation device 4 b comprises arotating equipment 68 supported by the horizontal wall 64 by theinterposition of a roller bearing 70 which allows the rotating equipment68 to rotate with respect to the supporting structure 62 and about theaxis 6 a. More particularly, the horizontal wall 64 supports an annularstructure 72 which is fixed to an upper face 64 a of the wall 64 andextends around the perimetric edges of the aperture 67. The annularstructure 72 carries a first annular element 72 a of the roller bearing72, while a second annular element 72 b of the roller bearing 72, lyingoutside the first, is fixed immovably to the rotating equipment 68. Therotating equipment 68 comprises a cup-shaped element in which an annularportion 74 coaxial with the axis 6 a is integral with a flat wall 75perpendicular to the axis 6 a. The flat wall 75 has a through hole 76whose function will be explained subsequently. The lower part of theannular portion 74 is connected by a plurality of screws 78 to a ringgear 77.

The ring gear 77 is engaged with a gear wheel 79 (FIG. 5) keyed on theoutput shaft of an electric motor 80 mounted on and projecting from thewall 64. Thus the electric motor 80 can cause the rotating equipment 68to rotate about the axis of rotation 6 a.

The traction device 3 b comprises a pair of traction pulleys 83 a, 83 bcarried by a supporting structure 82 (FIG. 5) of the rotating equipment68. The pulleys 83 a, 83 b are movable in rotation by the force of anelectric motor 85 mounted on and projecting from the wall 64 and locatedon the opposite side of the axis of rotation 6 a from the motor 80.

Each traction pulley 83 a, 83 b has an annular groove 84 a, 84 b (FIG.7) formed in an annular portion, preferably made from elastic material,of the pulley. A median plane P of the grooves 84 a, 84 b, also formingthe plane in which the pulleys 83 a, 83 b essentially lie, isperpendicular to the axes of rotation of the pulleys and passes throughthe axis of rotation 6 a.

As will be explained subsequently, the pulleys 83 a, 83 b are movablebetween a rest position in which they are spaced apart from each otherand an activation position in which the pulleys 83 a, 83 b areessentially tangential to each other and tangential to the axis ofrotation 6 a.

The transmission device 90, forming part of the traction device 3 b andenabling the motion to be transmitted from the motor 85 to the pulleys83 a, 83 b, will now be illustrated with particular reference to FIGS.5, 7 and 8. The transmission device 90 comprises a ring gear 92 which isconnected to the flat wall 75 with the interposition of a roller bearing93 which enables the ring 92 to rotate with respect to the rotatingequipment 68 and about the axis 6 a. More particularly, the flat wall 75supports an annular structure 94 which extends around the perimetricedges of the flat wall 75. The annular structure 94 carries a firstannular element 93 a of the roller bearing 93, while a second annularelement 93 b of the roller bearing 93, lying outside the first, is fixedimmovably to the ring gear 92. The ring gear 92 has an external toothing92 e (FIGS. 5 and 7) which is engaged with a gear wheel 95 keyed on theoutput shaft of the motor 85 and an internal toothing 92 i which isengaged with a gear wheel 97.

From the gear wheel 97 there extends a shaft 98 a (illustrated in FIG.8), which is positioned parallel to the axis of rotation 6 a and carriesat its upper end a first bevel gear 98 which is engaged at 90° with asecond bevel gear 99 integral with a first end of a shaft 100 which ispositioned perpendicularly to the axis of rotation 6 a and is carried byan extension 101 (FIG. 7) of the supporting structure 82. The shaft 100has a second end which emerges from the extension 101 and is integralwith a toothed pulley 103. The toothed pulley 103 is also connected, bymeans of a toothed belt 105, to a second toothed pulley 107 fixedimmovably to a first end of a shaft 110 which is parallel to the shaft100 and is carried by a central body 112 (FIG. 7) of the supportingstructure 82. The first end of the shaft 110 also carries a cup-shapedbody 115 provided with a toothed annular portion 116 whose function willbe explained subsequently. The shaft 110 extends into a cylindricalcavity 120 formed in the central body 112, and it is enabled to rotatewith respect to the central body 112 by a pair of bearings 123interposed between the shaft 110 and the walls of the cylindrical cavity120.

The shaft 110 extends into a cylindrical cavity 120 formed in thecentral body 112 and into a cylindrical tubular extension 122 whichextends from the central body 112 perpendicularly to the axis 6 a.

A cylindrical sleeve surrounding the shaft 110 and capable of keepingthe bearings 123 at a predetermined distance apart can be placed insidethe cavity 120.

A first oscillating element 125 of elongate shape (being shaped, forexample, as shown in FIG. 5 or as shown in the schematic example in FIG.8) is carried by the central body 112 and has a through hole 127 whichhouses the tubular extension 122, with the interposition of a ballbearing 128. The shaft 110 has a second end portion which projects fromthe tubular extension 122 and from the oscillating element 125, andwhich carries a gear wheel 130. The gear wheel 130 is engaged, by meansof a toothed belt 130 a, with a gear wheel 131 which is fixed on amedian portion of a shaft 132. The latter has a first end housed in asupporting bearing 134 carried by the oscillating element 125, and asecond end to which the traction pulley 83 a is fixed immovably. Theshaft 132 is also positioned parallel to the shafts 110 and 100 and isperpendicular to the axis of rotation 6 a.

The toothed annular portion 116 is engaged with a toothed annularportion 136 of a cup-shaped body 137 carried by a shaft 138 which ispositioned parallel to the shaft 110 and is carried by the central body112. In particular, the shaft 138 extends into a cylindrical cavity 140formed in the central body 112, and it is enabled to rotate with respectto the central body 112 by a pair of bearings 145 interposed between theshaft 138 and the walls of the cylindrical cavity 140.

In particular, the shaft 138 extends into a cylindrical cavity 140formed in the central body 112 and into a cylindrical tubular extension142 which extends from the central body 112 perpendicularly to the axis6 a and therefore parallel to the extension 122.

A cylindrical sleeve surrounding the shaft 138 and capable of keepingthe bearings 145 at a predetermined distance apart can be placed insidethe cavity 140.

A second oscillating element 150, shaped in the same way as the first(FIGS. 5 and 8) is carried by the central body 112 and has a throughhole 152 which houses, with the interposition of a ball bearing 153, thetubular extension 142.

The cup-shaped body 137 is also integral with a toothed pulley 158(FIGS. 7 and 8) which is engaged, by means of a toothed belt 160, with atoothed pulley 162 located at the first end of a shaft 164 carried bythe second oscillating element 150. A second end of the shaft 164emerges from the oscillating element 150 on the side opposite the pulley162 and carries the traction pulley 83 b. The shaft 164 is parallel tothe shaft 138 and perpendicular to the axis of rotation 6 a.

In use, the motion of the electric motor 85 is transmitted, by means ofthe gear wheel 95, to the ring gear 92 which rotates about the axis 6 a.The rotation of the ring gear 92 causes the rotation of the gear wheel97 which, because of the engagement between the bevel gears 96 and 99,causes the rotation of the pulley 103, which in turn transmits therotary motion to the cup-shaped body 115. The rotation of the cup-shapedbody 115 causes the rotation of the shaft 110, the gear wheel 130 andconsequently the traction pulley 83 a. The rotation of the cup-shapedbody 115 also causes the rotation of the cup-shaped body 137. In turn,the rotation of the cup-shaped body 137 is transmitted to the pulley 162which, by means of the rigid connection formed by the shaft 164, causesthe traction pulley 83 b to rotate.

The oscillating elements 125, 150 are driven by an opening device 170(FIG. 5) capable of keeping the pulleys 83 a, 83 b normally in theactivation position and capable of bringing them into the rest positionwhen so commanded.

With reference to FIGS. 4 and 5, the opening device 170 comprises a pairof rectilinear elements 171, 172 which have first end portions hinged toend extensions of the oscillating elements 125, 150 and second endportions hinged together about a pivot 173 located on an enlargedU-shaped portion of a cylindrical rod 175. The opening device 170 alsocomprises a tubular structure 180 which extends downwards from the wall75 parallel to the axis of rotation 6 a. The tubular structure 180 has alower end aperture closed by a plug 181 in which is formed a throughhole 182 coaxial with the tubular structure 180. The enlarged U-shapedportion is housed in the tubular structure 180 with the rod 175 whichengages in the aperture 182 and emerges from the plug 181. The portionof rod 175 which emerges from the plug 181 carries, at one of its ends,a ring 185 which forms a shoulder for a first end portion of a helicalspring 187 which is fitted around the rod 175 and has a second endportion bearing on the plug 181. The elastic force of the helical spring187 is capable of keeping the enlarged U-shaped portion bearing on theplug 181; in this position, the pivot 173 is in a lower position in thetubular structure 180, the rectilinear elements 171, 172 form an angle αof a few degrees between them (FIG. 5) and the oscillating elements 125,150 are placed adjacent to each other with the pulleys 83 a and 83 btangential to each other and tangential to the axis 6 a (if the elongatecylindrical element 5 is absent). When a force capable of opposing theelastic force of the spring 187 is applied to the rod 175, the enlargedU-shaped portion departs from the plug 181, being moved upwards;consequently, the pivot 173 is moved towards a median position of thetubular structure 180 and the rectilinear elements 171, 172 spreadapart, forming an angle β>α between them. Thus the oscillating elements125, 150 rotate and the pulleys 83 a and 83 b move away from each otherto enter the rest position.

Conveniently, the rod 175 is moved by means of a pneumatic actuator 190(FIGS. 4-6) carried by the wall 63 and provided with an output shaft 190a from which a bracket 193 extends transversely, this bracket beingconnected to a slide 194 movable with a reversible linear motion along avertical guide 195 carried by the metal wall 63 (FIG. 6).

The slide 194 carries a flat annular plate 196 perpendicular to the axis6 a. The annular plate 196 is movable between a lower rest position(illustrated in FIG. 4) in which it is spaced apart from the rod 175 andan upper position (not illustrated) of activation, in which the plate196 bears on the rod 175 and pushes it upwards, compressing the spring187.

In use, the actuator 190 is activated to bring the plate 196 into theactivation position, thus causing the upward movement of the rod 175,the compression of the spring 187 and the consequent rotation of theoscillating elements 125, 150; these elements rotate in oppositedirections, in such a way that the pulleys 83 a and 83 b move away fromeach other to enter the rest position.

The lower portion 5 i of the elongate cylindrical element 5 is thenpositioned between the pulleys 83 a, 83 b.

The actuator 190 is then turned off and the plate 196 moves to thepreceding rest position; consequently, the rod 175 returns downwardsunder the force of the spring 187, thus causing the oscillating elements125, 150 to rotate in such a way that the pulleys 83 a and 83 b approacheach other to enter the activation position.

In particular, both of the pulleys 83 a, 83 b are positionedtangentially to opposite sides of the elongate cylindrical element 5with respect to the axis of rotation 6, and press against the portion 5i (which is coaxial with the axis 6 a). The grooves 84 a, 84 b of thetraction pulleys come to bear on the opposite sides of the elongatecylindrical element 5 and exert an essentially constant pressure onthese, owing to the elastic force of the spring 187. Thus, an angularlyfixed connection is established between the pulleys 83 a, 83 b and theelongate cylindrical element 5.

The motor 85 is switched on, causing the pulleys 83 a, 83 b to rotate inopposite directions. Thus, these pulleys apply a tractive force to theelongate cylindrical element 5 which is stretched downwards with alinear motion. In its movement, the element 5 passes through theaperture 76 of the wall 75 and the aperture 67 of the wall 64.

At the same time, the motor 80 is switched on, causing the movableelement 68, and consequently the pulleys 83 a, 83 b, to rotate about theaxis 6 a.

In this way, the vertical plane P (FIG. 7) rotates about the axis ofrotation 6 a, and a twist about the axis 6 a is therefore applied to thecylindrical element 5 during its linear downward movement.

In FIG. 4, the part of the drawing in broken lines represents thesupporting structure 82 located in an angular position rotated through180° with respect to the position shown in solid lines.

1.-20. (canceled)
 21. A plant for producing an elongate cylindricalelement usable for a process of producing an optical fibre, comprising:a furnace capable of housing a preform of glass material and ofpartially fusing an extremity of said preform: a traction apparatuscomprising a traction device for providing a traction of said elongatecylindrical element from said partially fused extremity of said preformand along an axis of advance, wherein said traction device comprises atleast one pair of powered traction pulleys capable of being connected toat least one portion of said elongate cylindrical element and ofproviding said traction, said traction pulleys being in a common planeand movable between a rest position in which the first and secondpulleys are positioned away from each other and not in contact with theelongate element and an activation position in which the first andsecond pulleys approach each other and are in contact with the elongateelement; and a rotation device capable of applying a twist to saidpartially fused extremity of said preform about said axis,simultaneously with said traction.
 22. The plant according to claim 21,wherein said traction and twisting devices comprise at least one bodycapable of being connected to said portion of said elongate cylindricalelement and movable both with a rectilinear motion along said axis toprovide said traction and with a rotary motion about said axis toprovide said twist.
 23. The plant according to claim 22, wherein saidtraction device comprises a slide carrying said body and movable along aguide under the force of a first motor.
 24. The plant according to claim23, wherein said rotation device comprises a second motor connected tosaid body to cause said body to rotate about said axis of advance andwith respect to said slide.
 25. The plant according to claim 23, whereinsaid first motor has an output shaft connected to a screw and said slideis provided with a nut engaged with said screw to provide the movementof said slide along said guide under the force of said first motor. 26.The plant according to claim 24, wherein said body is integral with apulley connected by means of a belt to an output shaft of said secondmotor.
 27. The plant according to claim 21, further comprising anauxiliary traction apparatus in which an auxiliary traction device andan auxiliary rotation device are capable of operating alternately withsaid traction device and with said rotation device to provide,respectively, said traction and said twist of said elongate cylindricalelement.
 28. The plant according to claim 27, wherein said auxiliarytraction device and said auxiliary rotation device comprise at least oneauxiliary body which is capable of being connected to said portion ofsaid elongate cylindrical element and is movable both with a linearmotion along said axis of advance to provide said traction and with arotary motion about said axis of advance to provide said twist.
 29. Theplant according to claim 21, wherein said rotation device is capable ofproviding a rotary motion of said traction pulleys about said axis ofadvance to provide said twist of said elongate cylindrical element. 30.The plant according to claim 21, wherein said positioning devicecomprises a first oscillating body supporting a first traction pulleyand a second osciliating body supporting a second traction pulley, saidfirst oscillating body and said second oscillating body being angularlymovable about respective axes of rotation to place said traction pulleysin said activation position and in said rest position.
 31. The plantaccording to claim 21, wherein said positioning device comprises a pairof rectilinear elements having first end portions hinged tocorresponding oscillating elements and second end portions hingedtogether on an axially movable rod, said positioning device furthercomprising a striking wall movable under the force of at least oneactuator and capable of bearing on said rod to cause an axial movementof said rod, and consequently, a rotation of said oscillating elements.32. The plant according to claim 31, further comprising an elasticdevice interacting with said rod to keep said rod in a stable positionas a result of which said traction pulleys are located in saidactivation position.
 33. The plant according to claim 29, wherein saidrotation device comprises rotating equipment supporting said pair oftraction pulleys and rotationally movable under the force of a firstmotor about said axis of advance.